We have shown that several opioid peptides exert their hypotensive effects in the anesthesized rat via the activation of vagal afferent fibers. Pheochromocytomas contain considerable amounts of neuronal specific enolase (NSE), a biochemical marker for peptidergic neurons and APUD cells. However, the plasma concentration of NSE in pheochromocytoma patients is no higher than that found in normal individuals.